In the United States, the broadcast FM radio band occupies a band of frequencies extending from approximately 88 MHz to approximately 108 MHz. This band of frequencies lies between the frequencies allocated for broadcast television channel 6 and television cable channel 98. In addition, the National Weather Service broadcasts FM radio signals in a band of frequencies near 162.5 Mhz. Single-tuner multiband radio receivers having the capability to receive AM, broadcast FM, and National Weather Service radio signals, and television sound signals, are known from, for example, the Windsor Radio, model number 2239, FCC ID number BGK91F2239, manufactured in Hong Kong.
Television receivers having the capability to receive broadcast FM signals are known from the prior art. The Dumont model RA-119A television receiver, manufactured by Allen B. Dumont Laboratories, Passaic, N.J., is an example of a class of television receivers having a single tuner for receiving both television signals and broadcast FM radio signals. This single-tuner class of television receivers was manufactured during the years 1949 to 1952 inclusive, and utilized a continuous tuning arrangement for tuning signals having frequencies between 44 MHz and 217 MHz. This class of television receivers employed a so-called split sound IF system, that is, a separate sound IF channel tuned to 41.25 Mhz. The sound signals for both television and FM radio were demodulated directly from signals at the television sound IF frequency of 41.25 MHz.
Modern television receivers have abandoned the split-sound IF system in favor of the intercarrier sound system, which is less complex, less costly, and more reliable. The intercarrier-sound IF system takes advantage of the fact that the picture and sound carriers are held to close tolerances at the transmitter, ensuring that they are always separated by a constant 4.5 MHz. In an intercarrier-sound IF system, the sound IF signals are amplified along with the picture IF signals in a single IF amplifier. After amplification, the sound signals are converted to a 4.5 MHz intercarrier sound IF frequency by "beating" (i.e., heterodyning) the sound IF signals at 41.25 MHz against a signal at the picture carrier frequency of 45.75 Mhz. The television sound signals are then demodulated from the resulting 4.5 MHz intercarrier signal.
Modern intercarrier-sound type television receivers cannot recover and reproduce broadcast FM radio sound signals, because FM radio signals lack a signal at the picture carrier frequency required by the television receiver circuitry for use in recovering the sound signal. Therefore, as a result of adopting the intercarrier sound system, television manufacturers wanting to provide the capability of receiving broadcast FM radio signals, chose to add a separate FM radio having its own tuner. This may also have been done because modern television tuners commonly include tuned circuits (FM traps) for trapping out FM signals which may otherwise interfere with television video signal reception.
A typical FM trap for a television receiver is a tuned circuit exhibiting an amplitude vs. frequency characteristic having a single deep notch substantially centered on the FM band of frequencies, and sufficiently wide to attenuate FM signals throughout the FM band of frequencies. Not surprisingly, removing the FM trap degrades the video signal reproduction performance of the receiver when tuning television signals. This is deemed unacceptable because the receiver is primarily a television receiver, and only additionally an FM radio receiver.
Copending U.S. patent application Ser. No. 561,587 (Lehmann), discloses an FM trap for a television receiver which permits tuning of both television and FM radio signals by a single television tuner. The amplitude vs. frequency characteristic of the FM trap of Lehmann exhibits steep skirts (i.e., sharp rolloff) and a substantially flat band rejection region over substantially all of the FM broadcast band. As a result, the normally strong FM radio signals are attenuated uniformly over substantially the entire FM band. In addition to the uniform attenuation provided by the FM trap, a fixed gain control signal operates the television tuner at a lower gain setting when in FM reception mode, as disclosed in copending U.S. patent application Ser. No. 561,583 (Lehmann et al.).
Copending U.S. patent application Ser. No. 561,588 (Wignot et al.) discloses a television receiver in which during FM reception mode, the television receiver produces a video signal which when displayed identifies the FM station to which the receiver is currently tuned. Although the received FM radio signals are attenuated in the above-described trap, and amplified by the television tuner in a lower gain mode of operation, they nevertheless were found to be of a sufficient magnitude so as to adversely affect the synchronization of the display of the FM radio station identification message.